1. Field of the Invention
The present invention relates generally to the field of plastic nails. More particularly, it concerns an improved plastic nail pack for use in an automatic nailing machine. The nail packs are made of ultra-high strength plastic composites, for example, thermoplastic or thermoset materials.
2. Description of the Related Art
The use of machine-driven fasteners is widespread in industry, particularly in construction. Applications include furniture making, cabinet making, boat manufacture, roofing, dry wall installation, deck building, fence building and interior finish out, to name but a few. A major concern affecting the lifetime, quality and appearance of such applications is corrosion of the fastener. Galvanization of metal nails is the predominant method for reducing corrosion. Other methods include using aluminum or stainless steel or plastic-coated nails. All of these methods add considerable cost to the finished fastener, and do not completely prevent fastener corrosion.
Attempts have been made to produce nail fasteners from materials that do not significantly corrode, namely plastics. U.S. Pat. No. 2,510,693 to Green, Jun. 6, 1950 relates to fasteners made from a thermoplastic material having reinforcing fibers therein. U.S. Pat. No. 3,165,968 to Anstett, Jan. 19, 1965 describes a synthetic plastic nailing strip of, for example, a polyamide resin. U.S. Pat. No. 3,112,667 to Brentlinger, Dec. 3, 1963 relates to nails for use in dry wall construction, the nails having a recessed head and means for preventing flattening of the head when the nail is driven into a dry wall panel. U.S. Pat. No. 3,225,917 to Couch, Dec. 28, 1965 relates to a package of drive type fasteners for use in automatic nailing machines. U.S. Pat. No. 3,252,569 to Matthews, May 24, 1966 describes a plastic coated laminated nail having a reinforcing metallic wire core and a thermoplastic body. U.S. Pat. No. 3,348,669 to Powers, Oct. 24, 1967 describes formation of tools adapted to sever and drive individual fasteners from a stick supply of fasteners.
U.S. Pat. No. 3,492,907 to Hauck, Feb. 3, 1970 relates to a molded plastic tack strip adapted for use in a tack gun, the strip having a plurality of closely spaced tack shanks interconnected by severable webs. U.S. Pat. No. 3,813,985 to Perkins, Jun. 4, 1974 relates to coated fasteners, adhesive-coating compositions for fasteners, single-bevel, divergent point staples, and groups of staples united by an adhesive coating. U.S. Pat. No. 3,915,299 to Miyaoku, Oct. 28, 1975 describes a nail strip having side-to-side perforations in each nail permitting their alignment and a belt threaded through them for connection. The nail may be made of steel-reinforced plastics, such as methyl methacrylate, with waterproof, anti-moisture and anti-corrosion properties.
U.S. Pat. No. 4,456,123 to Russell, Jun. 26, 1984 relates to a method for attaching price tags to garments and for other joining applications using plastic fasteners dispensed through hollow, slotted needles. U.S. Pat. No. 4,664,733 to Masago, May 12, 1987 describes cohered fasteners, in particular, cohered nails, secured together in contiguous relation wherein separation of a fastener to be driven is facilitated. A group of wires is coated with an adhesive of thermoplastic resin and a second coating of nitrocellulose resin to form a strip for shaping into desired fasteners.
U.S. Pat. No. 4,826,381 to Kiriyama, May 2, 1989, incorporated by reference herein, relates to a continuous nail for automatic nailing machines. Each nail is injection molded with a thermoplastic resin e.g., polyamide resin, mixed with reinforcing materials, such as fine glass fiber, carbon fiber, etc. to improve the strength of the nail. The surface of the nail will be melted by frictional heat when it is driven for nailing. Kowa T Nail, a manufacture's brochure mentioned in patent '381, relates to a plastic nail, a plastic staple and a pneumatic nailer. The shape of the nail and low L/d necessitated the development of a special pneumatic tacker.
U.S. Pat. No. 4,971,503 to Barnell et al., Nov. 20, 1990 describes nail packs and clips used in automatic nailing guns. The nail package may contain plastic nails interconnected by integrally molded upper webs and lower webs. The plastic is a thermoplastic having high impact resistance and high tensile strength, such as Ultem, Nylon, A.B.S., polyester, polyphenoleneoxide and polycarbonate. U.S. Pat. No. 5,098,940 to Brooks, Mar. 24, 1992 relates to crystalline polyphthalamide component and a particulate thermotropic liquid crystalline polymer component in an amount sufficient to nucleate a melt of the polyphthalamide. The compositions are reported to be useful as injection molding compounds for production of electronic connectors, switch components, pump housings, valve components and under-the-hood automobile parts.
U.S. Pat. No. 5,153,250 to Sinclair, Oct. 6, 1992 describes compositions comprising (1) a polyphthalamide component comprising at least two recurring units selected from the group consisting of terephthalamide units, isophthalamide units and adipamide units and which, when filled with glass fibers, has a heat deflection temperature at 264 psi, according to ASTM D-648, of at least about 240.degree. C.; (2) about 10 to about 200 parts by weight reinforcing fibers per hundred parts by weight of the polyphthalamide component and (3) at least about 0.01 to about 5 parts by weight particulate talc per hundred parts by weight of the polyphthalamide component. The cited uses are the same as for U.S. Pat. No. 5,098,940.
U.S. Pat. No. 4,206,264 to Kurr relates to a group of polyester resins for coating fasteners. U.S. Pat. No. 5,149,237 to Gabriel et al. also relates to a coating for metal fasteners. The coating is a combination of two resins, a copolymer of preferably styrene and maleic anhydride and a thermoplastic resin, preferably vinyl acetate. U.S. Pat. No. 4,903,831 to Francis relates to an automatic hailer system which employs plastic ferrules which hold nails to form a strip.
The design of a nail for impact penetration is analogous to that of structural columns (Mechanics of Materials, 3rd ed., by Higdon et al., John Wiley & Sons, New York, 1976; and Design of Wood Structures, 3rd ed., by D. E. Breyer, McGraw-Hill, Inc., New York, 1993), with proper adjustment for the rate of load being applied, and assuming uniform stress in the nails during penetration. Adjustment of the rate of loading is accomplished by multiplication of a constant for impact loads. It serves to increase the allowable column stress compared to a load applied over a long period of time. The effect is the same for all plastic material types, therefore, is not applied in the analysis presented herein.
In ideal column design, two types of columns are considered: short and long. The former implies that the column will not buckle and its strength is related to the compression strength of the material. The latter presumes column instability, called buckling, as the only failure mechanism. In most situations, design of real columns, or nails, must consider the possibility of buckling (midsection of column) and crushing (ends of column). Actual column/nail behavior is defined by the interaction of the buckling and crushing modes of failure.
Whether the column is short or long is determined by the slenderness ratio which is the primary measure of buckling. The slenderness ratio is defined as ##EQU1##
Pure crushing can be considered the mechanism of failure for slenderness ratios less than about 2 or 3, and is measured by the compressive strength of the material. For slenderness ratios above about 130, buckling can be considered the single mode of failure. The maximum stress for pure buckling is defined by the Euler critical buckling stress for long slender members: ##EQU2##
F.sub.b =Euler critical buckling stress
E =flexural modulus
L =length
d =diameter (effective diameter)
To determine the actual maximum load the member can withstand without failure of either type, slenderness ratios between 3 and 130 should be analyzed using an empirical column formula to account for both buckling and crushing of the column member. Numerous empirical formulas exist for describing the maximum load for slenderness ratios between about 3 and 130. FIG. 3 depicts a maximum column load as a function of slenderness ratio (.sigma..sub.max is the compressive strength and E is the flexural modulus). The regions of pure crushing, pure buckling and combinations of crushing and buckling are shown. For simplicity, the materials of the examples presented herein are analyzed as ideal columns. In this case, the lines defining pure crushing and pure buckling define the maximum stress the member can withstand. Any of the empirical formulas could be chosen and applied to materials for detailed, individual analysis, but is unnecessary for comparisons between materials. It would have the same effect as multiplying all numbers by a constant; the relative values are still maintained.
Current plastic nails are not sufficiently strong to penetrate commodity woods such as white, yellow and treated pine, chip board or oak, and to be formed into a standard shape to fit existing nailers. In addition, current plastic nails are limited to a length/diameter ratio of less than about 16 with the overall length not generally exceeding one inch.